One of the more talked about custom firmwares looks to the one from Sveasoft. It touts SSH, telnet, TX power setting in the web interface, the ability to do client mode and tons more goodies. The are looking to fold in Rob Flickenger's NoCat Splash and Wondershaper soon.
Getting kismet_drone to run on the WRT54G (July 2004): http://toys.lerdorf.com/archives/20_Kismet_on_the_Linksys_WRT54G.html
Instructions for putting OpenWRT on the WRT.